I don’t get the concept of resting during your workout. Right now I am following a program that has 1 minute rest in between sets, while other exercises are 1-3 minutes. I am looking for bodybuilding only, not doing strength training. The thing I notice is I run out of gas on the next set, like I will do 12 reps on the first then only 8 on the 2nd and so on. What am I missing?